The present invention relates to a system for controlling a mechanical supercharger for an internal combustion engine to which is connected an automatic transmission.
The mechanical supercharger is driven by the engine to feed the cylinder forcibly with air or an air/ fuel mixture so that the engine output can be augmented by driving the supercharger. This supercharger is advantageous over the exhaust gas turbine in that it can effect the supercharging action even at a lower R.P.M. range and that it has a shorter turbo lag. The supercharger is driven when an increase in the engine output is demanded and when a smooth combustion is maintained even with more suction, because it is driven to increase the engine output especially by augmenting the suction. Generally in the prior art, therefore, the supercharger has its working range set on the basis of the engine load dictated by the throttle opening and the engine R.P.M. or its substitute vehicle speed, so that it is driven when the actual running state comes in that working range. Moreover, this working range is set to a lower throttle opening side when at a lower R.P.M., because a higher output is demanded, but to a higher throttle opening side according to the increase in the throttle opening when at a considerable or higher vehicle speed or engine N.P.M.
If, on the other hand, the range for switching the supercharger from ON to OFF and the range for switching it from OFF to ON are set identical, the supercharger is frequently turned ON/OFF by a slight change in the driving state such as the throttle opening or the engine R.P.M., to cause the so-called "hunting phenomenon". In the prior art, therefore, a hysteresis is set in the OFF-to-ON range and the ON-to-OFF range of the supercharger, and the boundary of the latter working range, i.e., the range for turning OFF the supercharger is set to a lower throttle opening side than that of the ON range. This technique for setting the hysteresis has been known in the art, as disclosed in Japanese Patent Laid-Open No. 195417/1987. In the invention, as disclosed in this Laid-Open, the hysteresis for controlling the ON/OFF of the supercharger at a higher gear ratio is set to have a small width.
As described above, the range for the supercharger to work is determined by using the throttle opening and the engine R.P.M. as its parameters. The supercharger is kept in its working state even if the actual throttle opening becomes smaller than that determining the working range but not over the hysteresis width. However, the working rage of the supercharger is set to the higher throttle opening side at a higher vehicle speed side, i.e., at a higher engine R.P.M. As a result, even the throttle opening within the supercharger working range at the lower R.P.M. side may go out of the working range at the higher R.P.M. side.
These situations are illustrated in FIGS. 3 and 4. In FIG. 3, a solid curve indicates the ON map of the supercharger at a predetermined gear stage set in an automatic transmission, and a broken curve indicates an OFF map of the same. This OFF map is set with a predetermined hysteresis (e.g., -5 degrees) with respect to the ON map. At tills gear stage, the (not-shown) accelerator pedal is depressed to augment the throttle opening, and the vehicle runs at a constant acceleration with the throttle opening being kept. The running state of this case is indicated by an arrow in FIG. 3. If, therefore, this running state changes to cross the OFF map, as indicated by the broken curve in FIG. 3, it comes into the OFF range of the supercharger so that the supercharger is interrupted at the instant across the OFF map.
As a result, even if the throttle opening makes no change, as shown in FIG. 4, the supercharger changes into its inoperative state at an instant to when the running state crosses the OFF map, to cause reductions of the engine torque and the acceleration and a temporary drop of the engine R.P.M. These changes occur when the driver makes no change in his maneuver so that shock and discomfort are felt by the driver.